Art of electrodeposition of copper



Patented Nov. 26, 1946 ART OF ELEGTRQDEPOSITION OF COPPER Ernest D. Wilson, Worcester, Mass., assignor to Arthur D. Little, 1210., Cambridge, Mass., a corporation of Massachusetts No Drawing. I Application March 4, 1943,

Serial No. 478.009

32 Claims- (Cl. 204-52) I This invention relates to the art of electrodeposition of copper. Substantially all commercial copper plating processes involve the use of either copper cyanide type baths or acid copper baths. Both types or bath, however. are subject to various disadvantages. I

While acid copper baths have the advantage of high cathode efllciency and simplicity, they cannot be usedcommercially for plating iron, steel and zinc articles because, with such a bath, copper is deposited on iron, steel or zinc by immersion. As a result, it is practically impossible to obtain satisfactory electro-depositions on such metals.

With copper cyanide plating baths, copper is not deposited by immersion. Consequently, more or less satisfactory, adherent electro-deposits can be obtained. While such baths are used extensively for plating iron, steel and zinc articles, they are objectionable for other reasons. For example, they have low cathode efficiency and they are highly poisonous. They are relatively unstable and their composition must be carefullycontrolled and frequently re-adjusted. Also, they tend to pick up carbon dioxide from the atmosphere, whereby they acquire too high a carbonate content.

Moreover whether copper cyanide baths or acid copper solutions are used, the plated articles must be buffed. One reason for this is that the copper deposit is dull and buffing is necessary to obtain the necessary high polish or bright surface before the article can be nickel plated or otherwise finished. Secondly, the copper deposits or film almost invariably contains many pinholes. Unless the plate is bulied to cause the copper to flow over or into these pinholes, the basis metal is apt to corrode.

The builing operation is commercially disadvantageous because a considerable amount of hand labor is involved and the cost of buiiing usually greatly exceeds the cost of the plating itself. Furthermore, the bumng operation removes some copper and the initial deposit must consequently, be thicker than is desired for the finished article.

It is an object 01' the present invention to provide a ,copper plating solution and method of plating such that the objectionable features 01 copper cyanide and acid copper baths are avoided.

More particularly, it is an object of the invention to provide a plating solution and method of plating such that copper is not deposited by immersionon iron or zinc; and such that the resulting electro-deposition is of a character that does not require bufling.

Further objects of the invention are to provide an improved adhesion of the plating to the base metal, as well as a lustrous finish to the surface 2 c of the electro-deposited plating so as to render it suitable for subsequent plating directly with out any necessity for the mechanical polishing operations, such as bufling, and consequent loss of the plating metal.

Other objects and advantages of the invention will be apparent to those skilled in the art from .the following description.

In accordance with the present invention, I form a; copper plating bath by admixing a suitable copper salt, an alkylene amine and water.

. Various copper salts may be used, for example,

copper sulphate, copper acetate or copper carbonate. I have found, however, that copper sulphate gives the best results. I have also found that what are now considered the best results are obtained by using ethylene diamine as the I what diiierent results as to the color of copperdepositeda Alkali salts of hydroxy acids, such as sodium lactate and Rochelle salts, may also be used with reasonably satisfactory results. I have found, however, that the best results are obtained with lactic acid.

j In conjunction. with the presence of the alkyl-' ene polyamine in the electrolytic bath, it is found that the addition of such hydroxy acid or one of its alkali metal salts, and more especially the addition of lactic acid, contributes greatly to the adherence of the plating deposited to the surface of the base metal, to which it is applied in accordance with the present invention. For ex,- ample, in the electroplating of steel with copper,

.it was found that while thin coatings could be applied satisfactorily without such addition, the

. tendency of thick coatings of copper to peel from" the base metal was substantially overcome by making such addition to the electrolyte. In consequence platings so provided will withstand impact tests which actually puncture the base metal, without peeling off.

I have also discovered that improved results ,are obtained if some boric acid is added to the bath.

' The addition of boric acid to the electrolyte is not essential to the obtainment of the improvement in the adherence of-the plating applied to the base metal. But when added to the electrolyte as prepared in accordance with this invention, it

does not interfere with such improvementvand,

supplemental thereto, has the very desirable and advantageous efiect of imparting a lustrous bril liancy and dense structure to the surface of the electro-plating deposit so that it may serve directly for the application of subsequent electroplating operations and deposits without further preparation or treatment. It thus effects a great saving in operations, equipment and material, as well as time.

The proportions of the several starting materials may vary within a considerable range. The invention may be satisfactorily carried out, however, according to the following example:

I form a bath with the following starting materials (commercial form) Copper sulphate (CuSO4.5HzO) pounds 50 Ethylene diamine (70% do 55 Lactic acid (85%) do 11 Boric acid do 16% Water to make gallons 100 4 necessary in the proportion 01' the other starting materials.

If the hydroxy acid is not used, the amount of alkylene amine may be reduced accordingly. For example, if the lacticacid is omitted from the formula above set forth, the amount of ethylene diamine may be reduced by about 3% pounds. Generally speaking, substantially the same ratio may be followed. Also the pH of the solution should be adjusted to 8.2 to 8.5 for the best results. This adjustment may be attained by means of sulphuric acid or amine. While a bright plate may be obtained without the use of the hydroxy acid, its use is recommended because it results in a copper deposit having better adherent qualities.

As indicated from the example above given. with reference to the copper salt present in the electrolyte, the hydroxy acid, such as lactic acid, is added in approximately the proportion 01' onehalf 9. mol and the boric acid in equimolecular proportions. But these may be varied somewhat plating. While the operating conditions may vary, I have found that operation at room temperatures, at a current density of about 10 amperes per square foot, gives satisfactory results.

With the plating bath described, there is no,

deposit of copper by immersion on iron or zinc so that the difliculty inherent in acid copper solu-- tions is avoided. The solution does not have the present invention is that articles copper plated in accordance therewith do not require bufling. The bath described gives a bright plate that requires no bufllngas a precedent to nickel plating or other finishing. Moreover, the deposit obtained is extremely fine grained with no-pinholes so that bufiing is not necessary to protect the basis metal. As a result, there is a marked saving in cost and inconvenience and the serious objections to both copper cyanide and acid copper baths are avoided.

A further advantage of the bath described is that it has an extremely high throwingv power, even greater than. that or a copper cyanide bath. While the chemical reactions involved are not entirely understood and need not be understood to carry out the invention, it is believed that the amine forms with the sulphate a copper complex having a ratio of 2 molecules of amine to one of copper sulphate. This complex is so stable that I the pH of the solution may be lowered to 2 without decomposing it. It is believed, also, that the hydroxy acid forms a copper complex of unknown formula- The function of the. boric acid is not known. A good deposit may be obtained without it but when it is present it appears to result in a brighter deposit. If the boric acid is omitted, no change is as indicated by the followingranges of proportions which are satisfactory, with reference to one mol of C11S04.5H2O in solution for example:

, Mole Lactic acid .3to .9 Citric acid .3 to .4 Rochelle salt .2 Boric acid .7 to 1.75

In general, the alkylene polyamine may advantageously be present in excess of equimolecular proportions, with reference to the copper salt, but it may vary considerably (so long as it is not too greatly reduced) without appreciably affecting the results to be obtained with the electrolyte in question. I

Accordingly the present invention, by the application of the hydroxy acids of the class indicated or their soluble salts, especially to electrolytes containing an alkylene polyamine, makes possible the ready and assured formation of dense, firmly adherent electroplatings of copper and also the provision of a highly lustrous. brilliant, electroplated surface when boric acid is also added, which requires no subsequent treatment such as bufflng in order to make the surface satisfactory as a finished product and hence without the minute marks or she'en which is likely to accompany the most expert mechanical workmanship It appears generally to be most favorable, under actual operating conditions and for best results, to employ the member of the hydroxy acid group, such as lactic acid in particular, in a molecular proportion relative to one mol of the copper salt in solution in'the electrolyte, of from about onequarter to about one mol, either when used alone or when used in conjunction with the boric acid. With regard to the boric acid a wider range of proportions of from about one-half mol to about two mols, relative to one mol of the copper salt, such as copper sulphate in solution may be most effectively employed for the purposes indicated of securing both an improved adherent plate and at the same time providing a highly polished surface suitable for subsequent direct plating thereon of other metals such as silver.

This is a continuation-in-part of my copending application Serial No. 185,809, filed January 20, 1938. i

I claim:

1. A. process of electroplating that comprises .electro-depositing copper as a bright, smooth,

firmly adherent deposit from an undivided cell 1 said salt, and lactic acid.

containing an aqueous electrolyte consisting essentially of-a soluble copper salt and a sufilcient quantity of an alkylene polyamine to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids.

2. A process of electroplating that comprises electro-depositing copper as a bright, smooth,'

firmly adherent deposit from an undivided cell containing an aqueous electrolyte consisting essentially of a soluble copper salt and a. sufiicient quantit oi an alkylene polyamine to form a complex with said salt, boric acid, and a compound selected from the group consisting of lactic, citric and tartaric acids and the. alkali metal salts of such acids.

' plex with said salt and a'compound selected from the group consisting of. lactic, citric and tartaric acids and the alkali metal salts of such acids, in

approximately one-half molecular ratio and boric acid in approximately equal molecular proportion, with reference to the copper salt.

- 3. A process of electroplating that comprises sentially of copper sulphate and a sufiicient quantity of ethylene diamine to form a complex with said salt and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts-of such acids. I

4. A process of electroplating that comprises electro-depositing copper as a bright, smooth, firmly adherent deposit from an undivided cell containing an aqueous electrolyte consisting essentially of copper sulphate and a sufiicient quantity of ethylene diamine to form a complex with said salt, boric acid, and a compound selected from the group consisting of lactic,,citric and tartaric acids and the alkali metal salts of. such acids.

.electro-depositing copper as .a bright, smooth,

firmly adherent deposit from an undivided cell containing an aqueous electrolyte consisting essentially of a soluble copper salt and a suificient quantity of an alkylene polyamine to form a complex with said salt, and boric and lactic acids.

7. A process of electroplating that comprises electro-depositing copper as a bright,-smooth,

firmly adherent deposit from an undivided cell containing an aqueous electrolyte consisting essentially 01 copper sulphate and a sufllcient quantity of ethylene diamine to form acomplex with 8. A process of electroplating that comprises electro-depositing copper as a bright, smooth,

, firmly adherent deposit from an undivided cell firmly adherent, deposit, from an undivided vcell "containing an aqueous electrolyte consisting-essentially of a soluble copper salt and a suflicient quantity of an alkylene polyamine to form a complex with said salt and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in approximately one-half molecular ratio with reference to the copper salt.

10. A process of electroplating that comprises two molsof boric acid, 'and'lactic acid inthe' molecularproportion of from about one-fourth 11. A process of electroplating that comprises *electro-depdsiting copper as a bright, smooth,

firmly adherent deposit from an undivided cell containing an aqueous electrolyte consisting essentially of a soluble copper salt and a sufficient quantity of an alkylene polyamine to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts 01' such acids in v the molecular proportion of from about onefourth mol to about one mol, relative to said coppersalt.

12. A process of electroplating that comprises electro-depositing copper as a bright, smooth, firmly adherent, deposit from an undivided cell containing an aqueous electrolyte consisting. essentially of a, soluble copper salt and a sufllcient quantity of an alkylene polyamine to form a complex with said salt, about one-half mol to about two mols of boric acid, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about onei'ourth mol to about one mol, relative to said cop per salt., l

13. A process of electroplating that comprises .electrodepositing copper as a bright, smooth, firmtially of a soluble copper salt and a sufllcient I quantity of an alkylene polyamine to form a complex with said salt, and lactic acid in the molecularproportion of from about one-fourth mol to about one mol, relative to said copper salt.

containing an aqueous electrolyte consisting essentially of a'soluble copper salt and a sufiicient' quantity of an alkylene polyamine to form a complex'with said salt, aboutbne-hali mol to about salt.

15. A process of electroplating that comprises electro-depositing copper as'a bright, smooth, firmly adherent deposit from an undivided cell contalning an aqueous electrolyte consisting esfsentially of copper sulphate and a sufilcient quantity of ethylene diamine to form a complex with-said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about onefourth mol to about one mol, relative to said assume mole of boric acid, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about onefourth mol to about one mol, relative to said copper sulphate.

17. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufficient quantity of an alkylene polyamine to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids.

18. An aqueous electrolyte for use in the electro-deposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufficient quantity of an alkylene polyamine to form a complex with said salt, borlc acid, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids.

19. An aqueous electrolyte for use in the electro-deposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of copper sulphate and a sufllcient quantity of ethylene diaminc to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids.

20. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of copper sulphate and a suihcient quantity of ethylen diamine to form a complex with said salt, boric acid, and a. compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids.

21. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufficient quantity of an alkylene polyamlne to form a complex with said salt, and lactic acid.

22. An aqueous electrolyte for use in the e1ectrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufiicient quantity of an alkylene polyamine to form a complex with said salt, and boric and lactic acids.

23. An aqueous electrolyte for use in the electrodeposition of copperas a bright, smooth, firmly adherent deposit that consists essentially of copper sulphate and a suiiicient quantity of ethylene diamine to form a complex with said salt, and lactic acid. I

24. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firm- 1y adherent deposit that consists essentially of copper sulphate and a sumcient quantity oiethylene diamlne to form a complex with said salt, and toxic and lactic acids. 1

25. An aqueous electrolyte for use in the electrcdeposition of copper as a bright, smooth, firm- 1y adherent deposit that consists essentially of a soluble copper salt, a sufiicient quantity of an alltylene polyamine to form a complex with said salt, and a compound. selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids, in approximately one-half molecular proportion, with retierence to the copper salt.

26. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt, a suflicient quantity of an alkylene polyamine to form a complex with said salt and a compound, selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids, in approxi mately one-half molecular proportion and boric acid in approximately equal molecular proportion, with reference to the copper salt.

27. An aqueous electrolyte for use in the electrodepositlcn of copper as a bright, smooth, flrmly adherent deposit that consists essentially of a soluble copper salt and a sufllcient quantity of an alkylene polyamine to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about one-fourth mol to about one mol, relative to said copper salt.

28. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufficient quantity of an alkylene pclyamine to form a complex with said salt, about one-half mol to about two mols oi borlc acid, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about one-fourth mol to about one mol, relative,to said copper salt.

29. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of a soluble copper salt and a sufilcient quantity of an alkylene polyamine to form a complex with said salt, and lactic acid in the molecular proportion of from about one-fourth mol to about one mol, relative to said copper salt. 1

30. An aqueous electrolyte for use in the elec trodeposition of copper as a bright, smooth, firmly adherentdeposit that consists essentially of a soluble copper salt and a suflicient quantity of an alkylene polyamine to form a complex with said salt, about one-half mol to about two mols of boric acid and lactic acid in the molecular proportion of from about one-fourth mol to about one mol, relative to said copper salt.

31. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firmly adherent deposit that consists essentially of copper sulphate and a sufllcient quantity of ethylene diamine to form a complex with said salt, and a compound selected from the group consisting of lactic, citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about one-fourth mol to about one mol. relative to said copper sulphate.

32. An aqueous electrolyte for use in the electrodeposition of copper as a bright, smooth, firm ly adherent deposit that consists essentially of copper sulphate and a sufllcient quantity of ethylene diamine to form a complex with said salt, about one-half mol -to about two mols of boric acid, and a compound selected from the group consisting of lactic. citric and tartaric acids and the alkali metal salts of such acids in the molecular proportion of from about one-fourth mol to about one mol, relative to said copper sulphate.

ERNEST D. WILSON. 

